Question: In geometry class, the girl to boy ratio is $3$ to $7$. If there are a total of $30$ students, how many boys are there?
Solution: A ratio of $3$ girls to $7$ boys means that a set of $10$ students will have $3$ girls and $7$ boys. A class of $30$ students has $3$ sets of $10$ students. Because we know that there are $7$ boys in each set of $10$ students, the class must have $3$ groups of $7$ boys each. There is a total of $21$ boys in geometry class.